Hot-melt adhesives which are useful in such applications as the bonding of labels, tapes and decals to various substrates are well known in the art. Such adhesives are normally specially adapted for use on certain kinds of substrates. At the present time, however, I am unaware of amorphous olefin-based hot-melt adhesives which have the versatility of being useful in bonding to a variety of substrates such as glass, metal and polymers. Also, the hot-melt adhesives according to the present invention have a good balance of high and low temperature bond quality, and the combination of good tack and low viscosity for ease of application. Furthermore, the hot-melt adhesives according to the present inventin have low color, low odor when molten, good thermal stability and less tendency of bleed-through when compared to known conventional hot-melt adhesives.
The adhesives according to this invention are generally applied by a hot-melt applicator which includes a reservoir for the adhesive, heating means for melting the adhesive and maintaining it in a molten state, and means for applying the adhesive for a particular end use.
To be suitable for use on typical hot-melt labeling equipment, the adhesive should have several key characteristics. Among these are low viscosity at typical application temperatures of 150.degree.-180.degree. C., resistance to stringing, aggressive tack, long open time, and good specific adhesion to typical container and label substrates. Other desirable properties are low color and odor, low tendency to bleed through label stock, and good thermal stability. In the past, hot-melt adhesives useful for labeling have been commonly based on ethylene-vinyl acetate copolymers or synthetic block copolymer rubbers which are diluted with rosins, waxes, and plasticizers to yield the desired machining characteristics. Such formulations are generally high in color and odor, and exhibit significant changes in viscosity on exposure to heat for extended periods. While polyolefin-based adhesives exhibit low color and odor, and good thermal stability, those available to date generally lack the tack and adhesion necessary to bond a variety of label stocks to various substrates, and have not been available with the low viscosity necessary for machining on hot-melt labeling lines. Therefore, it would be an advance in the state of the art to provide a polyolefin-based adhesive having the necessary low-melt viscosity, good tack and specific adhesion, and low color and odor to be desirable in bonding or sealing labels, tapes, decals, etc., to provide bonds having good strengths at both low and high temperatures.
One patent of interest is U.S. Pat. No. 4,567,223. This patent discloses a hot-melt adhesive comprising a blend of at least one modified (such as by reaction with an unsaturated polycarboxylic acid) propylene/C.sub.4 to C.sub.10 higher 1-olefin copolymer having an acid number of at least 5, a tackifying resin, and a high density, low viscosity polyethylene wax having a molecular weight of at least about 1000. The present inventin differs from the hot-melt adhesive of U.S. Pat. No. 4,567,223 in that the adhesive of the present invention uses an unmodified olefin polymer or copolymer. It further differs from this patent in that both a hydrocarbon tackifier and a rosin tackifier are required. The U.S. Pat. No. 4,567,223 discloses in Example 8 that low density (i.e., 0.925) polyethylene wax is not satisfactory in that invention.